Plasma immersion ion implantation has been used for simultaneous implantation of carbon and nitrogen into austenitic stainless steel X5CrNi189 at 400 degrees celsius. Duplex layer structures are formed with a nitrogen-rich layer close to the surface and a carbon-rich layer at greater depth. The influence of different gas compositions on the structure and phase composition of the layers is examined and compared with results from pure nitrogen and methane. The results provide further information about 'expanded austenite' (S-phase) which is formed during the low temperature nitriding of austenitic stainless steels.